Java tutorial
/* * Copyright 2002-2004 The Apache Software Foundation * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.apache.commons.collections.buffer; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; import java.util.AbstractCollection; import java.util.Arrays; import java.util.Collection; import java.util.Iterator; import java.util.NoSuchElementException; import org.apache.commons.collections.BoundedCollection; import org.apache.commons.collections.Buffer; import org.apache.commons.collections.BufferOverflowException; import org.apache.commons.collections.BufferUnderflowException; /** * The BoundedFifoBuffer is a very efficient implementation of * Buffer that does not alter the size of the buffer at runtime. * <p> * The removal order of a <code>BoundedFifoBuffer</code> is based on the * insertion order; elements are removed in the same order in which they * were added. The iteration order is the same as the removal order. * <p> * The {@link #add(Object)}, {@link #remove()} and {@link #get()} operations * all perform in constant time. All other operations perform in linear * time or worse. * <p> * Note that this implementation is not synchronized. The following can be * used to provide synchronized access to your <code>BoundedFifoBuffer</code>: * <pre> * Buffer fifo = BufferUtils.synchronizedBuffer(new BoundedFifoBuffer()); * </pre> * <p> * This buffer prevents null objects from being added. * <p> * This class is Serializable from Commons Collections 3.1. * * @since Commons Collections 3.0 (previously in main package v2.1) * @version $Revision: 1.11 $ $Date: 2005/01/22 00:48:22 $ * * @author Avalon * @author Berin Loritsch * @author Paul Jack * @author Stephen Colebourne * @author Herve Quiroz */ public class BoundedFifoBuffer extends AbstractCollection implements Buffer, BoundedCollection, Serializable { /** Serialization version */ private static final long serialVersionUID = 5603722811189451017L; /** Underlying storage array */ private transient Object[] elements; /** Array index of first (oldest) buffer element */ private transient int start = 0; /** * Index mod maxElements of the array position following the last buffer * element. Buffer elements start at elements[start] and "wrap around" * elements[maxElements-1], ending at elements[decrement(end)]. * For example, elements = {c,a,b}, start=1, end=1 corresponds to * the buffer [a,b,c]. */ private transient int end = 0; private transient boolean full = false; /** Capacity of the buffer */ private final int maxElements; /** * Constructs a new <code>BoundedFifoBuffer</code> big enough to hold * 32 elements. */ public BoundedFifoBuffer() { this(32); } /** * Constructs a new <code>BoundedFifoBuffer</code> big enough to hold * the specified number of elements. * * @param size the maximum number of elements for this fifo * @throws IllegalArgumentException if the size is less than 1 */ public BoundedFifoBuffer(int size) { if (size <= 0) { throw new IllegalArgumentException("The size must be greater than 0"); } elements = new Object[size]; maxElements = elements.length; } /** * Constructs a new <code>BoundedFifoBuffer</code> big enough to hold all * of the elements in the specified collection. That collection's * elements will also be added to the buffer. * * @param coll the collection whose elements to add, may not be null * @throws NullPointerException if the collection is null */ public BoundedFifoBuffer(Collection coll) { this(coll.size()); addAll(coll); } //----------------------------------------------------------------------- /** * Write the buffer out using a custom routine. * * @param out the output stream * @throws IOException */ private void writeObject(ObjectOutputStream out) throws IOException { out.defaultWriteObject(); out.writeInt(size()); for (Iterator it = iterator(); it.hasNext();) { out.writeObject(it.next()); } } /** * Read the buffer in using a custom routine. * * @param in the input stream * @throws IOException * @throws ClassNotFoundException */ private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { in.defaultReadObject(); elements = new Object[maxElements]; int size = in.readInt(); for (int i = 0; i < size; i++) { elements[i] = in.readObject(); } start = 0; full = (size == maxElements); if (full) { end = 0; } else { end = size; } } //----------------------------------------------------------------------- /** * Returns the number of elements stored in the buffer. * * @return this buffer's size */ public int size() { int size = 0; if (end < start) { size = maxElements - start + end; } else if (end == start) { size = (full ? maxElements : 0); } else { size = end - start; } return size; } /** * Returns true if this buffer is empty; false otherwise. * * @return true if this buffer is empty */ public boolean isEmpty() { return size() == 0; } /** * Returns true if this collection is full and no new elements can be added. * * @return <code>true</code> if the collection is full */ public boolean isFull() { return size() == maxElements; } /** * Gets the maximum size of the collection (the bound). * * @return the maximum number of elements the collection can hold */ public int maxSize() { return maxElements; } /** * Clears this buffer. */ public void clear() { full = false; start = 0; end = 0; Arrays.fill(elements, null); } /** * Adds the given element to this buffer. * * @param element the element to add * @return true, always * @throws NullPointerException if the given element is null * @throws BufferOverflowException if this buffer is full */ public boolean add(Object element) { if (null == element) { throw new NullPointerException("Attempted to add null object to buffer"); } if (full) { throw new BufferOverflowException("The buffer cannot hold more than " + maxElements + " objects."); } elements[end++] = element; if (end >= maxElements) { end = 0; } if (end == start) { full = true; } return true; } /** * Returns the least recently inserted element in this buffer. * * @return the least recently inserted element * @throws BufferUnderflowException if the buffer is empty */ public Object get() { if (isEmpty()) { throw new BufferUnderflowException("The buffer is already empty"); } return elements[start]; } /** * Removes the least recently inserted element from this buffer. * * @return the least recently inserted element * @throws BufferUnderflowException if the buffer is empty */ public Object remove() { if (isEmpty()) { throw new BufferUnderflowException("The buffer is already empty"); } Object element = elements[start]; if (null != element) { elements[start++] = null; if (start >= maxElements) { start = 0; } full = false; } return element; } /** * Increments the internal index. * * @param index the index to increment * @return the updated index */ private int increment(int index) { index++; if (index >= maxElements) { index = 0; } return index; } /** * Decrements the internal index. * * @param index the index to decrement * @return the updated index */ private int decrement(int index) { index--; if (index < 0) { index = maxElements - 1; } return index; } /** * Returns an iterator over this buffer's elements. * * @return an iterator over this buffer's elements */ public Iterator iterator() { return new Iterator() { private int index = start; private int lastReturnedIndex = -1; private boolean isFirst = full; public boolean hasNext() { return isFirst || (index != end); } public Object next() { if (!hasNext()) { throw new NoSuchElementException(); } isFirst = false; lastReturnedIndex = index; index = increment(index); return elements[lastReturnedIndex]; } public void remove() { if (lastReturnedIndex == -1) { throw new IllegalStateException(); } // First element can be removed quickly if (lastReturnedIndex == start) { BoundedFifoBuffer.this.remove(); lastReturnedIndex = -1; return; } int pos = lastReturnedIndex + 1; if (start < lastReturnedIndex && pos < end) { // shift in one part System.arraycopy(elements, pos, elements, lastReturnedIndex, end - pos); } else { // Other elements require us to shift the subsequent elements while (pos != end) { if (pos >= maxElements) { elements[pos - 1] = elements[0]; pos = 0; } else { elements[decrement(pos)] = elements[pos]; pos = increment(pos); } } } lastReturnedIndex = -1; end = decrement(end); elements[end] = null; full = false; index = decrement(index); } }; } }